A(1) Field of the Invention
The invention generally relates to a television transmission system comprising a transmitter station, which includes an encoding station, and comprising a receiver station, which includes a decoding station, for transmitting picture signals in a digital format via a transmission medium. More particularly, the invention relates to a television transmission system of the type in which redundancy-reducing encoding is effected in the encoding station and redundancy-restoring decoding is effected in the decoding station.
Such a television transmission system may form part of a television broadcasting system, in which case the encoding station forms part of the television broadcasting transmitter and each TV receiver includes a decoding station. The transmission medium is the atmosphere in this case. Such a television transmission system may also be a video recording system, in which case the transmission medium is, for example, a video tape or a compact disc.
A(2) Description of the Prior Art
As is generally known, a television picture is completely determined by three picture signals. These may be the three primary colour signals R, G, B or, which is the same, one luminance signal Y and two colour difference signals U and V, also referred to as I and Q. These colour difference signals will hereinafter be referred to as CHR(1) and CHR(2).
As is also generally known, a sampling frequency is associated with each digital signal. This means that the samples constituting this digital signal occur at this sampling frequency. In practice, each sample is represented by a number comprising a given number of bits, For the current digital television transmission systems the sampling frequency is standardized at 13.5 MHz. The sampling is performed in such a manner that 720 samples for each of the three picture signals are obtained for each visible line of the picture. If the number of visible lines per television picture is assumed to be 576 and the number of bits per sample is assumed to be eight, approximately 10 Mbits are to be transmitted for each television picture. At 25 pictures per second this means a bit rate of approximately 250 Mbit/second. This requires a transmission channel having a bandwidth of approximately 125 MHz. In practice, such transmission channels are usually not available. For example, in a video recorder the transmission channel in this case formed by the write head, the tape and the read head has a maximum bandwidth of 30 MHz.
To be able to transmit a television picture in a digital format through a transmission channel having a considerably limited bandwidth, it is common practice (see for example References 1 and 2 in the next section C) to use the luminance signal Y and the two colour difference signals CHR(1) and CHR(2) as picture signals, instead of the three primary colour signals. This choice is made because the luminance signal has a considerably larger signal-energy content than each of the two colour difference signals. If this luminance signal is sampled for its transmission at a frequency f.sub.Y, each of the two colour difference signals can be sampled at a lower frequency f.sub.CHR. More particularly, f.sub.Y is an integral multiple N of f.sub.CHR. As already stated, f.sub.Y =13.5 MHz and N is chosen to be b 2 for the current television transmission systems. Consequently, the bit rate can be reduced to a value of approximately 167 Mbit/second, which is 2/3 of the original bit rate. Reference 1 states that N may even be chosen to be 3 without serious loss of quality. It also states that the number of colour difference samples to be transmitted can be halved without any noticeable loss of quality by transmitting during one of two successive picture lines the samples of one of the two colour difference signals. For example, during a first picture line the samples of the colour difference signal CHR(1) are transmitted only and during a subsequent picture line the samples of the colour difference signal CHR(2) are transmitted only. The so-called vertical decimating filters with which this is realised (referred to as "Vertikales Chrominanzfilter" in Reference 1) and which have a decimation factor R which in this case is equal to two, thus produce digital auxiliary colour difference signals CH(1) and CH(2). The encoding station of this known transmission system has also an input circuit which receives the picture signal Y and the two auxiliary colour difference signals CH(1) and CH(2) and which has two auxiliary picture signal outputs at which a first and a second auxiliary picture signal occur. More particularly, the first auxiliary picture signal is equal to the luminance signal and the second auxiliary picture signal is constituted by a time-division multiplex of the samples of the one auxiliary colour difference signal CH(1) of the one picture line and the samples of the other auxiliary colour difference signal CH(2) of the subsequent picture line.
For a further bit rate reduction the samples of the luminance signal Y occurring at the first auxiliary picture signal output are also applied to a first redundancy-reducing encoding circuit and the samples of the multiplex signal occurring at the second auxiliary picture signal output are also applied to a second redundancy-reducing encoding circuit. Both redundancy-reducing encoding circuits are differential pulse code modulators.
Each redundancy-reducing encoding circuit produces a channel signal which is applied to the transmission medium by means of an output circuit. These signals applied to the transmission medium will be referred to as transmission medium signals.
It is to be noted that it is known per se to realise redundancy reduction in a way other than by means of differential pulse code modulation, for example, by performing a Q*Q forward transform. In that case all those samples of a picture signal forming one complete picture are considered as elements (pixels) of a matrix. This matrix is divided into sub-matrices of Q by Q matrix elements and this sub-matrix is written as the sum of Q.sup.2 orthonormal Q*Q basic matrices each with its own weighting factor (coefficient). In such a redundancy reduction a part of the said weighting factors is transmitted instead of the original samples.
Yet another method of realising redundancy reduction is the use of differential pulse code modulation combined with a Q*Q forward transform. Such a combination is also referred to as hybrid encoding (see, for example Reference 3).
For regaining the original picture signals Y, CHR(1) and CHR(2) the receiver station of the television transmission system described in Reference 1 has an input circuit which can be coupled to the transmission medium and which regenerates the original two channel signals, starting from the information (transmission medium signals) present at this transmission medium. Each channel signal is applied in a decoding station to an individual redundancy-restoring decoding circuit whose operation is inverse to the said redundancy-reducing encoding circuits and each of them supplies a regenerated local auxiliary picture signal. These two auxiliary picture signals are converted by means of an output circuit into the original luminance signal Y and the original two auxiliary colour difference signals CH(1) and CH(2), which signals occur at separate outputs of this output circuit. The two auxiliary colour difference signals are subjected to a vertical interpolation operation so as to generate the original colour difference signals CHR(1) and CHR(2).
As is known, much research has been done in the field of high-definition television in the last few years. High-definition television picutres comprise, for example 1250 lines of which 1152 are visible and carry picture information. Moreover, the aim is a higher horizontal resolution in combination with a higher aspect ratio, namely 16:9; in the current television picutres this ration is 4:3. It is to be noted that the aspect ratio of a television picture is the ratio between the horizontal and the vertical dimension of the picture. Transmission of picture signals for such picutres in a digital format implies in the first place that the required sampling frequencies must be chosen to be considerably higher than is common practice in the current television transmission systems. For obtaining, inter alia a sufficiently high horizontal resolution, a sampling frequency of 54 MHz is recommended. If such a high sampling frequency were used in the television transmission system described in Reference 1, it would mean that the one redundancy-reducing encoding circuit receives samples (namely those of the luminance signal) at a rate of 54 MHz and the other at a rate of 18 MHz.
When using such a transmission system in consumer apparatus such as, for example digital video recorders, the aim will be a high degree of integration. If the integrated circuits are formed in the currently most advanced integration technique, which is known under the name of CMOS, a high internal processing rate, namely up to 30 MHz is possible at a high integration density. When integrating the redundancy-reducing encoding circuit receiving samples at a rate of 54 MHz, this internal processing rate of 30 MHz is absolutely insufficient. It is to be noted that it is possible to integrate an encoding circuit by means of CMOS techniques and to realise an internal processing rate of 54 MHz or more, but this is at the expense of the integration density. In fact, this density considerably decreases with an increasing processing rate. The required chip surface and hence the price of the chip increase considerably.